This invention relates generally to acetabular prosthetic devices and more particularly to an improved acetabular shell liner wherein the liner has a variable geometry rim surface.
Artificial implants, including hip joints, shoulder joints and knee joints, are widely used in orthopedic surgery. The human hip joint acts mechanically as a ball and socket joint, wherein the ball-shaped head of the femur is positioned within the socket-shaped acetabulum of the pelvis. In a total hip joint replacement, both the femoral head and the surface of the acetabulum are replaced with prosthetic devices.
A first general class of hip prosthetic devices included an acetabular component in which the head of a prosthetic femoral component was intended to articulate relative to the acetabular component. Initial designs featured an acetabular component with a thin bearing surface, or liner, which interfaced with a large femoral component head. This design allowed for good range of motion and a low incidence of dislocation or subluxation of the femoral component head, but the thin liners proved to wear poorly, requiring replacement.
Acetabular components generally comprise an assembly of a shell and a liner. The completed acetabular component is configured to be received and fixed within the acetabulum of a pelvis. Generally, a metal shell and a polymeric liner are used to form the component, however, the liner may be made of a variety of materials, including but not limited to, polyethylene, ultra high molecular weight polyethylene, metal, and ceramic materials. Shells may typically be comprised of titanium, stainless steel, cobalt chromium, alloys thereof, or any other appropriate biocompatible material.
Typically the shell has a hemispherical shape and features an outer, convex surface and an inner, concave surface that is adapted to receive the shell liner. The liner fits inside the shell and has a convex and concave surface. It acts as a bearing element in the acetabular component assembly. The convex surface of the liner corresponds to and sits in the inner concave surface of the shell or acetabulum, and the concave surface receives the head of a femoral component.
The internal concave surface of the liner is characterized by features relative to an axis through a center of the concave surface. This axis may or may not be aligned with the central axis of the shell. In a typical liner, the concave surface has a hemispherical geometry and may be referred to as the internal diameter. In such liners, the geometry is characterized by features that are concentric to an axis that runs through the center of the internal diameter.
However, liners may have differing forms and geometries. For example, liners may be made in a one-piece configuration or they may consist of more than one piece. When in place, they may lie flush with the shell opening, they may have portions that extend above the shell opening, or they may completely extend above the shell opening, depending upon the intended use of the liner. The uses and possibilities of various liners and their relation to embodiments of the invention will be described in more detail below.
The femoral component generally comprises a spherical or near-spherical head attached to an elongate stem with a neck connecting the head and stem. It may be a one-piece unit or may comprise a modular or multi-piece unit. In use, the elongate stem is located in the intramedullary canal of the femur and the spherical or near-spherical head articulates in the internal diameter of the liner.
Currently, a hip joint prosthesis may comprise an acetabular component having a thicker liner and a femoral component having a smaller sized head than the initial designs. Acetabular designs that include thicker liners provide more bearing support and less surface area for wear, but they may present problems with dislocation and subluxation, as well as reduced range of motion, due to the smaller head size. Thus, one of the critical concerns in designing total hip joint replacement components is how to design the components to minimize contact of the neck of the femoral component with the rim of the liner during articulation, thus reducing rim contact-induced subluxation, dislocation, and wear, while allowing a maximum desired range of motion. There are a variety of acetabular liners available for use in hip replacement procedures that seek to address the issues of limited range of motion, rim-contact wear, and dislocation or subluxation.
For example, the standard, non-anteverted liner, also called a flat, neutral, or zero degree liner, has a wide rim, or impingement, surface. Typically, the center of rotation of the femoral head on a standard liner is concentric with the acetabular shell. This type of standard liner is used to provide a broad range of motion. Use of this liner requires optimal positioning of the acetabular component in the acetabulum in order to provide the required range of movement for a patient. While standard liners allow a broad range of motion, if malpositioned, they present an increased possibility of dislocation. To address this problem, a high wall liner may be used.
In contrast to standard liners, high wall liners, also known as shouldered, lipped, or elevated liners, employ an extended, elevated portion over a segment of the periphery of the liner internal diameter in order to increase coverage of the femoral head, and thus reduce the likelihood of dislocation and aid in reduction of the head should subluxation occur. The use of high wall liners may be beneficial in cases of tenuous stability in order to avoid dislocation. See e.g. T. Cobb, et al., The Elevated-Rim Acetabular Liner in Total Hip Arthroplasty: Relationship to Postoperative Dislocation, Journal of Bone and Joint Surgery, Vol. 78-A, No. 1, January 1996, pp. 80-86.
Another type of liner (that may, in some instances, also be considered a high wall liner) is an anteverted liner. Anteverted liners angle or rotate the central axis of the internal diameter of the liner relative to the central axis of the shell. Anteverted liners shift the contact area on the head of the femoral component in order to improve hip joint stability and decrease the risk of dislocation. However, use of an anteverted liner may reduce allowed range of motion.
Another type of liner that may be used to prevent recurrent dislocation in a high risk patient is a constrained liner. This type of liner has an elevated portion over the entire periphery of the liner internal diameter. The elevated rim can reduce the chance for dislocation or subluxation of the femoral implant, but can also limit its range of motion. In short, high wall liners of all designs (including anteverted and constrained liners) may reduce the arc of motion to contact in the direction of the elevated rim segment without a corresponding increase in motion in the opposing direction. Thus, there is a substantial loss of overall range of motion compared to a standard (or flat or zero degree) liner. This reduction in range of motion makes the rotational positioning or clocking of these designs in the acetabulum particularly important in order to reduce rim contact with the neck of the articulating femoral component and potential acceleration of polyethylene wear at the rim as a result of this contact.
In general, a liner has a rim where the inner surface meets the outer surface of the liner. The rim may be in the form of an edge, a chamfer, a radius, or a surface. Some of the available liners have a constant geometry relieved rim surface around the circumference of the internal diameter of the liner. While a relieved rim surface increases range of motion, the constant geometry may not optimize the possible range of motion because it may not be correlated to the cross-section of the femoral component during a condition of femoral component neck-liner contact. At this point, the femoral component is said to be in an impingement condition with the liner.
Prosthesis range of motion has been evaluated in the past by creating a cone that defines the limits of motion to contact, or impingement angles, for the prosthesis, as described in Thornberry, et al., The Effects of Neck Geometry and Acetabular Design on the Motion to Impingement in Total Hip Replacement, A Scientific Exhibit at the 1998 AAOS Meeting, New Orleans, La., 1998, the entire contents of which are hereby incorporated by reference. The size of the cone depends on the design of the components. Varying the orientation of the components allows a surgeon to shift the direction of the cone. In a successful component placement, the cone is positioned so that adequate range of motion for the patient is provided. The base of the cone provides information for flexion, extension, adduction, and abduction. The direction of flexion-extension, as well as abduction-adduction, can be drawn as a line on the base of the cone. The point where the line intersects the cone is the maximum motion of prosthesis in the respective direction. Designs that provide adequate range of motion generally correlate with good clinical results. See e.g. B. McGrory, et al., Correlation of Measured Range of Hip Motion Following Total Hip Arthroplasty and Responses to a Questionnaire, Journal of Arthroplasty, Vol. II, No. 5, 1996.
Thus, there is a need for a method of forming an acetabular shell liner that provides optimization of the maximum range of motion and minimum interference with the femoral component neck. There is a further need for a constrained liner that can limit the possibility for dislocation, but also provide a greater range of motion and limit impingement of the femoral component. There is a further need for methods that can be used to form such liners.